This application relates to arterial spin labeling (ASL) in magnetic resonance imaging (MRI).
Imaging through MRI techniques is well known. In essence, a typical MRI technique produces an image of a selected body part of an object under examination by manipulating the magnetic spins of hydrogen atoms in the body part such as fat and water molecules and processing measured responses from the magnetic spins. A MRI system may include hardware to generate different magnetic fields for imaging, including a static magnetic field along a z-direction to polarize the magnetic spins, gradient fields along mutually orthogonal x, y, or z directions in a xyz coordinate system to spatially select a body part for imaging, and an RF magnetic field (B1) to manipulate the spins. MRI techniques may be used to capture the functional changes in body parts or tissues such as the brain perfusion.
One commonly-used technique for functional MRI is in vivo imaging by arterial spin labeling (ASL), where the arterial blood is tagged by magnetic inversion using RF pulses applied to a plane or slab of arterial blood proximal to the tissue of interest. Images are typically acquired with and without prior tagging of arterial blood and are subtracted to produce images that are proportional to perfusion. This magnetic tagging allows for the imaging of blood flow without the administration of dyes or other imaging agents. Hence, ASL provides non-invasive tagging in MRI measurements.
Methods based on such ASL, however, are spatially selective and thus require the tagging be done at a plane or slab close to the target issues. Notably, there is a transit delay (Δt) for the delivery of tagged blood to the target tissues. This delay can be on the order of the T1 time of blood and is probably the largest source of errors in the quantitation of cerebral blood flow using ASL in the human brain. More specifically, the time for delivery of the tagged blood to the target tissues by blood flow is not short compared to the lifetime of the tracer (T1). The T1 of blood is approximately 1.3 seconds, while the delivery time in the brain is about 0 to 2 seconds in healthy subjects and may reach 5 to 10 seconds in pathological cases. Such heterogeneity of delivery time usually arises from variations in the distances and flow velocities along the vascular tree from the tagging location to the tissues of interest. In the important clinical applications such as stroke, the collateral routes of blood circulation can lead to a delivery time much larger than the time T1 and thus can cause false positive findings of low perfusion when in fact perfusion is present via collateral routes of circulation.
Such spatial selectivity and the associated delay are undesirable and are present in various available ASL methods including EPISTAR, PICORE, FAIR, QUIPPS and continuous ASL techniques. In pulsed techniques, a slab of tissue containing arterial blood is tagged, while in the continuous techniques blood flowing through a defined plane is tagged upon traversal of the plane. Pulsed and continuous techniques based on spatially dependent tagging are generally susceptible to delivery time related artifacts.